碧口地体同位素地质年代学及其意义

ISOTOPIC GEOCHRONOLOGY AND ITS SIGNI-FICANCE IN BIKOU TERRAIN,CHINA

碧口群分布于陕、甘、川交界处摩天岭区,为一套海相火山-沉积岩系,变质可达绿片岩相。它呈东宽西窄的楔形地体分布于扬子板块、松潘-甘孜褶皱系与秦岭褶皱系等三个一级大地构造单元之间,均以断裂相接,是一个中晚元古代运移地体,于晋宁运动晚期拼贴于扬子板块北缘。笔者用锆石U-Pb及^(40)Ar-^(39)Ar等多种同位素地质定年方法,对碧口地体主要成岩时代、变质时代、中酸性岩侵入时代及构造运动时代等主要地质事件进行了定年研究,初步建立了碧口群同位素地质年表,为研究碧口地体构造演化提供了重要的同位素地质年代数据。

The Bikou Group, as a large wedge -wide to the east and narrow to the west—occurred amoung Yangtse plate, Songpan-Ganzhi Fold system and Qinling Fold system with fault boundaries, is a middle to Pro terozoic(migrated)terrain. It was accreted on the north margin of Yangtse plate in late Jinning movement. Bikou Group consists of marine volcanic rocks and sedimentary rocks, and is metamorphosed by chlorite facies. Problems, such as stratigraphic sequence and forming age etc., have not been solved before because of the complex structural pattern and the lack of biological fossils. The authors have studied the ages of main geological events by using zircon U-Pb, ^(40)Ar/^(39) Ar and Rb-Sr geochronological methods and divided the previous broad sense Bikou Group into three exclusive parts: (1)the Yudongzi Gr., the late Archean basement of Bikou Group, whose main phase of metamorphic age, determined by U-Pb concordency diagram of zircon from migmatized amphibolite, is 2657±9 Ma and whole rock K-Ar dilution age of amphibolite dike that intruded into the Yudongzi Gr. is 2038±30 Ma.(2) the Bikou Gr. (narrow-sense division of this paper)of which is subdivided into two Subgroups and further into five Formations. The diagenetic age of upper rock group in lower Subgroup, determined by ^(41)Ar-^(39)Ar of pyrite, is 1565±16 Ma; the regional metamorphic age of upper Subgroup by K-At of rhodonite is 1044±16Ma. Comparing spilite-keratophyre sequence of lower Subgroup in Bikou with Great wall Formation in the North China, and stromatolite-bearing dolomite sequence of the upper Subgroup with Jixian Formation, it is estimated that the age of Bikou Gr. is limited to 1800-1000Ma. (3)The overlying upper Sinian to lower Cambrian strata, whose boundary age between Linjiang Formation of upper Sinian and Gangou Formation of lower Cambrian, determined by whole rock Rb-Sr isochion, is 581±20Ma. In addition, the authors have studied the ages of intermediate to acid intruding rocks through comparative analysis of ^(49)Ar-^(39)Ar, zircon U-Pb and K-At. By using biotite and hornblende ^(40)Ar-^(39)Ar methodes, we respectively determined that the age of Yangba biotite monzonitic granite is 1235 Ma and Tongchang quartz dioriteis 1335 Ma. Based on isotopic geochronology and combining with comprehensive analysis for the era of surrounding rock, contact ralationship and location in tectonic magma zone of Bikou ancient arc-island system, four phases of intermediate to acid intrusive rock in Bikou Terrain have been distinquished: (1) Wuling-sibu epoch of intrusive rock (1800-1000 Ma); (2) Jinning epochaof intrusive rock (1000-800Ma); (3) Calidonian intrusive rock; and (4) Indo-Chinese to Yanshan epoch of intrusive rock. To sum up, Bikou Terrane is the complex region that is surperimposed by polyphase tectonic thermal envents in its evolutional history. The ages of single zircon U-Pb is considered as a reformation age, and the ^(40)Ar-^(39)Ar or zircon U-Pb concordancy diagram ages are approximate to diagenetic ages of intrusive bodies. Through the detailed study of isotopic geochronolopy, we initially divided the evolution history of Bikou Terrain into three tectonic cycles: (1) Late Archean-Early Proterozoic era, the period of formation of the crystalline basement (2657-2000 Ma); (2) Middle proterozoic era, the period of formation and evolution of Bikou Terrain (1800-800 Ma); (3) Sinian period and later, sedimentary covers unconformably overlying on Bikou Terrain and intercontinental orogenic movements with magma activities.